Python--描述符

描述符是描述类A的实例对象a 在另一个类B中实例描述对象a, 再实例化B的对象b后，b就可以通过操作a来达到一些A中定义好的方法 如get set del。

简单例子：

class MyDecription:
    def __get__(self, instance, owner):  #self是自己的MD实例对象 instance是被描述的实例对象test owner是test类
        print('getting', instance, self, owner)
    def __set__(self, instance, value):
        print('setting', self, instance, value)
    def __delete__(self, instance):
        print('delling', self, instance)


class Test:
    x = MyDecription() #描述符



test = Test()
test.x
print('-'*30)
test.x = 'x-man'
print('-'*30)

输出：

getting <__main__.Test object at 0x00000190199BACA0> <__main__.MyDecription object at 0x00000190199BBFD0> <class '__main__.Test'>
------------------------------
setting <__main__.MyDecription object at 0x00000190199BBFD0> <__main__.Test object at 0x00000190199BACA0> x-man
------------------------------

 机制例子：逐行解读

class MyProperty:
    def __init__(self, fget=None, fset=None, fdel=None):    #运行顺序第4 self = x
        self.fget = fget   
        self.fset= fset
        self.fdel = fdel

    def __get__(self, instance, owner):
        return self.fget(instance)  #x.getx(c)

    def __set__(self, instance, value):    #运行顺序第8
        self.fset(instance, value)         #运行顺序第9 x.setx(c, 5)

    def __delete__(self, instance):
        self.__delete__(instance)


class C:
    def __init__(self):
        self._x = 1                #运行顺序第2     c._x = 1

    def getx(self):                #运行顺序第6     返回_x = 1
        return self._x

    def setx(self, value):
        self._x = value            #运行顺序第10 c._x = 5

    def delx(self):
        del self._x

    x = MyProperty(getx, setx, delx)    #运行顺序第3



c = C()                            #运行顺序第1 
print(c._x)                        #运行顺序第5 激活get方法
c.x = 5                            #运行顺序第7 调用修饰函数的set方法
print(c._x)                        #运行顺序第11 调用c的getx方法 返回c._x

摄氏度华氏度互相转换的例子： 

class Celssius:
    def __init__(self, value = 26.0):                        #3
        self.value = float(value)                            #4 cel.value = float(value)

    def __get__(self, instance, owner):                        #13
        print(self.value, '调用Cel的get')
        return self.value

    def __set__(self, instance, value):                   #7 cel, temp, 30
        print('调用cel的set')                             #8
        self.value = float(value)                         #9 cel.value = float(30)


class Fahrenheit:
    def __get__(self, instance, owner):                   #11   fah, temp, Temperature
        print(instance.cel * 1.8 + 32)                    #12 temp.cel 
        return instance.cel * 1.8 + 32                    #14

    def __set__(self, instance, value):
        instance.cel = (float(value) - 32) / 1.8 

class Temperature:

    cel = Celssius()                                                    #2
    fah = Fahrenheit()                                                  #5


temp = Temperature()                                                    #1
temp.cel = 30                                                           #6 调用修饰的set
temp.fah                                                                #10 获取fah的get
temp.fah = 90
temp.cel